1. Reduction-Division Genetic Recombination
MEIOSIS
Reduction-Division
Genetic Recombination

2. Meiosis
The form of cell division by which GAMETES, with HALF the number of CHROMOSOMES, are produced.
DIPLOID (2n)  HAPLOID (n)
Meiosis is SEXUAL reproduction.
TWO divisions (MEIOSIS I and MEIOSIS II).

3. Meiosis Sex cells divide to produce GAMETES (sperm or egg).
Gametes have HALF the # of chromosomes.
Occurs only in testes or ovaries.

4. Males n=23 n=23 2n=46 sperm haploid (n) Meiosis II human sex cell
diploid (2n)
n=23
Meiosis I

5. females n=23 n=23 2n=46 Haploid (1n) human egg sex cell
Meiosis II
2n=46
human
sex cell
diploid (2n)
n=23
Meiosis I
Polar Bodies (die)

6. Interphase I Similar to mitosis interphase.
CHROMOSOMES (DNA) replicate in the S phase
Each duplicated chromosome consist of two identical SISTER CHROMATIDS attached at their CENTROMERES.
CENTRIOLE pairs also replicate.

7. Interphase I Nucleus and nucleolus visible. Nucleus chromatin
cell membrane
nucleolus

8. Meiosis I (four phases)
Cell division that reduces the chromosome number by one- half.
Four phases:
a. Prophase I
b. Metaphase I
c. Anaphase I
d. Telophase I
Prophase I

9. Prophase I Longest and most complex phase (90%). Chromosomes condense.
Homologous chromosomes come together to form a tetrad.
Tetrad is two chromosomes or four chromatids (sister and non-sister chromatids).

10. Non-Sister Chromatids-HOMOLOGS
Homologs contain DNA that codes for the same genes , but different versions of those genes
Genes occur at the same loci

11. Prophase I
Homologous chromosomes
sister chromatids
Tetrad

12. Homologous Chromosomes
Pair of chromosomes (maternal and paternal) that are similar in shape and size.
Homologous pairs (tetrads) carry GENES controlling the SAME inherited traits.
Each locus (position of a gene) is in the same position on homologues.
Humans have 23 pairs of homologous chromosomes:
a. First 22 pairs of autosomes
b. Last pair of sex chromosomes
LOCI

13. Homologous Chromosomes
eye color
locus
hair color
Paternal
Maternal

14. Sex Chromosomes
XX chromosome - female
XY chromosome - male

15. MEIOSIS I
Meiosis I
Homologs separate

16. Prophase I Nucleus & Nucleolus disappear Spindle forms
Chromosomes coil & Synapsis (pairing) occurs
Tetrads form
centrioles
spindle fiber
aster
fibers
TETRAD

17. Metaphase I
Shortest phase
Tetrads align on the equator.

18. Homologs line up at equator or metaphase plate
Metaphase I
Homologs line up at equator or metaphase plate OR

19. Anaphase I Homologous chromosomes separate and move towards the poles.
Sister chromatids remain attached at their centromeres.

20. Anaphase I
Homologs separate

21. Telophase I Each pole now has haploid (1n) set of chromosomes.
Cytokinesis occurs and two haploid daughter cells are formed.

22. Telophase I
cytokinesis

23. Sister Chromatids Separate
MEIOSIS II
Sister Chromatids Separate
Meiosis II

24. Meiosis II No Interphase II or very short No DNA Replication
Remember: Meiosis II is similar to mitosis

25. Prophase II Same as Prophase in mitosis Nucleus & nucleolus disappear
Chromosomes condense
Spindle forms

26. Metaphase II Same as Metaphase in mitosis
Chromosomes (not homologs) line up at equator

27. Anaphase II
Same as Anaphase in mitosis
SISTER CHROMATIDS separate

28. 1n Sperm cell fertilizes 1n egg to form 2n zygote
Telophase II
Same as Telophase in mitosis.
Nuclei and Nucleoli reform, spindle disappears
CYTOKINESIS occurs.
Remember: FOUR HAPLOID DAUGHTER cells are produced.
Called GAMETES (eggs and sperm)
1n Sperm cell fertilizes 1n egg to form 2n zygote

29. Telophase II

31. Question:
A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

32. Answer:
10 chromosomes (haploid or 1n)

33. Fertilization The fusion of a sperm and egg to form a zygote.
A zygote is a FERTILIZED EGG
n=23
egg
sperm
n=23
2n=46
zygote

34. Question:
A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

35. Answer:
10 chromosomes